Fired carbon-containing magnesia bricks are known. These bricks are produced by mixing classified magnesia granules and flours with a binder which ensures sufficient strength for handling and subsequently pressing and firing at from 1500 to 1800° C. The fired bricks are then impregnated with pitch at from 150° C. to 200° C. under reduced pressure. As a result of pyrolysis during use of the bricks, carbon packets which hinder infiltration of, for example, slag constituents or gases are formed from the pitch. The internal bonding of the bricks results from sintering bridges formed between the magnesia grains during firing. It is possible to achieve a residual carbon content of about 2% by weight in these bricks.
In addition, unfired, carbon-bonded magnesia bricks are used mainly as linings in steel equipment in the steel industry. These bricks are produced by mixing classified magnesia flours with pitch as binder at from 100° C. to 200° C. and pressing the mixtures hot. This is followed by heating at from 250° C. to 350° C., resulting in formation of a carbon bonding phase in the form of a carbon framework from the pitch. Disadvantages are that unburned hydrocarbons, which are hazardous to health, can be liberated after installation of the bricks and during heating in the steel production apparatus, and undesirable pressure softening of the bricks occurs.
To avoid these disadvantages and to avoid the energy-consuming hot pressing, synthetic resins in the form of liquid phenol resoles or phenol-novolak solutions are used as binders and pressing is carried out cold at room temperatures using a hardener or hot at temperatures in the range from room temperature to below 100° C. Curing is subsequently carried out at from 120° C. to 200° C., resulting in crosslinking of the resin to form an infusible resin lattice which likewise results in carbon bonding.
The carbon bonding, which ensures handling strength of the bricks after pyrolysis or heat treatment, is retained even when the bricks are used at high temperatures. Apart from the carbon bonding, the carbon, which largely fills in the interstices between the magnesia grains, also drastically hinders infiltration when the bricks come into contact with slags and gases.
Furthermore, magnesia-carbon bricks are known. Graphite is mixed together with the binder which affects the carbon bonding into the batch for producing these bricks, as a result of which a higher carbon content in the bricks and, associated therewith, increased hindering of infiltration can be achieved. Pitch or synthetic resin is used as the binder.
To achieve carbon bonding, it is desirable for very substantial graphitization of the carbon to occur by means of pyrolysis. This is achieved more readily when using pitch than when using synthetic resin. The graphitization of synthetic resin can be considerably improved by the use of graphitization aids as described in EP 1 280 743 B1.
The carbon ensures mainly the bonding of the unfired bricks and a reduction in wear as a result of infiltration being hindered. In addition, the thermal shock resistance and the thermal conductivity, in particular, are increased and the thermal expansion is reduced.
Bonding of the bricks results essentially from the adhesion between the carbon framework of the binder and the magnesia grains and also, in particular, from the cohesion within the carbon framework. In this context, it is known that weakening of the microstructure in the interior of the bricks can occur as a result of redox reactions during operation at high temperatures. The carbon participates in these redox reactions are partly burned out (see Gerald Routschka, Taschenbuch “Feuerfeste Werkstoffe”, 3rd edition, Vulkan-Verlag, Essen, p. 172, paragraph 3 to p. 173, FIG. 2).